Complimentary metal oxide semiconductors (CMOS) diodes are commonly used for sensing images in cameras and other video or photo devices. Recently, CMOS devices have been improved by using backside illumination (BSI). Generally, photolithography processes deposit CMOS structures on the top side of silicon wafer or other substrate. Early CMOS devices gathered light from the top, the same side where the CMOS structure was applied. Metal interconnects deposited on the top surface of the CMOS substrate can block portions of the photosensitive CMOS structure, degrading the picture quality and individual CMOS sensitivity. BSI is the collection of light from the backside of the CMOS substrate, with the CMOS Sensor deposited onto the top side of the substrate, and then the substrate abraded or otherwise thinned to allow light to pass though the substrate and affect the photosensitive region of the CMOS device. Ideally the substrate thickness is reduced so that light may enter the backside of the CMOS device and strike the photosensitive region of the CMOS, eliminating obstruction and interference during image capture from deposited structures and metal interconnects.
Shallow trench isolation (STI) is a technique used in CMOS image sensor circuit devices to isolate structures and regions. A physical trench may be etched around the CMOS structure into the substrate to provide physical separation of the CMOS structure from adjacent structures. Additionally, the STI trench may be filled with an oxide such as silicon dioxide (SiO2). Structures in the periphery, such as row select transistors, reset transistors, or the like, may also have STI trenches etched into the substrate to isolate them from the active photosensitive CMOS structure. STI permits CMOS sensors to operate more accurately, preventing spurious electrons from striking the CMOS photosensitive region and simulating a photon being detected. The STI trench assists in preventing spurious electrons from affecting the CMOS sensor by eliminating a straight line path between the CMOS sensor and any other structures.